Yatsenyuk's shirt, and much of his face, was left dripping in deep-green liquid after he and his first deputy, Oleksandr Turchynov, were confronted by a pair of assailants carrying spray bottles and more than a dozen vials of a Soviet-era topical antiseptic known as "zelyonka."

Zelyonka, which is frequently translated as "brilliant green," has been used for years in Ukraine, Russia, and elsewhere to treat minor cuts and scrapes, often giving its wearers a slightly ghastly, zombie-like aspect.

But in recent months, zelyonka has also enjoyed a resurgence as an extremely distinctive form of nonlethal aggression in Ukraine's Euromaidan protests. Pro-government activists—mainly young people and the so-called "titushky" enforcement brigades—have repeatedly used the antiseptic to attack prominent members of the political opposition and even protesters themselves.

Zelyonka, a diluted solution of a triarylmethane dye, is toxic when ingested but relatively harmless when it comes in contact with skin. Its effectiveness, therefore, is in its intense staying power—in its pure form, after all, it's used to dye silk and wool. It can take days for the emerald-green stains to fade, and its victims inevitably face an immediate, post-splash flash of cameras as the liquid drips down their face.

Zelyonka attacks have become so routine that opposition leaders have learned to take it in stride. Yatsenyuk, the right side of his face almost entirely green, cheerfully smiled to the nearby clutch of journalists without so much as dabbing his cheek with a tissue.

And Yuriy Lutsenko, Tymoshenko's former interior minister—who, like Tymoshenko, was jailed in a politically charged case before being pardoned last year—calmly continued a late December speech in Kharkiv after three men and a woman sprayed the crowd with brilliant green. Zelyonka has become such a sensation that Ukrayina television dedicated an entire news item to the attack on Lutsenko, noting that his wife, journalists, and TV cameras were also left drizzled in the stuff.

The attacks seem to focus mainly on Batkivshchyna members, including Arsen Avakov and Oleksandr Kirsh, who was sprayed—again, in Kharkiv—while campaigning in the 2012 parliamentary elections. He was hospitalized after suffering a chemical burn to his eye.

The Tymoshenko associate who has probably received the worst of the zelyonka attacks is her lawyer, Serhiy Vlasenko, who has been splash-attacked numerous times. Most recently, he was attacked on December 6 inside a Kharkiv courtroom hearing additional tax-evasion charges against Tymoshenko, who is already serving seven years for abuse of power. He was also doused with a jug of brilliant green in July 2012.

Perpetrators of zelyonka attacks have been prosecuted, but usually receive no more than a light punishment for disorderly conduct.

There are signs, however, that zelyonka can serve as a threat of more sinister punishment to come. Automaidan activist Dmytro Bulatov, who says he was kidnapped and tortured for more than a week before being released, said before his abduction that toys streaked with zelyonka had been thrown at his house. Bulatov has three children.